The invention relates to a process for the production of 1,2 dichloroethane, hereinafter referred to as “EDC”, which primarily serves as an intermediate product in the production of monomer vinyl chloride, hereinafter referred to as “VCM”, which, in turn, is used to produce polyvinyl chloride (PVC), and the invention also relates to a facility for running the said process. Hydrogen chloride (HCl) is obtained when EDC is reacted to produce VCM. Hence, the preferred method of producing monomer vinyl chloride (VCM) from ethene C2H4 and chlorine Cl2 such that a balance is maintained between the hydrogen chloride (HCl) produced and consumed in the various reactions, which is substantiated as follows:

The process for the production of VCM with an adequate HCl balance—hereinafter referred to as “balanced VCM process”—comprises the following process steps:                a direct chlorination in which one portion of the required EDC is produced from ethene C2H4 and chlorine Cl2 and made available as the so-called pure EDC, the recovery of the reaction heat developed in this direct chlorination being the main aim of the invention;        an oxichlorination in which the remaining portion of the required EDC is produced from ethene C2H4, hydrogen chloride HCl and oxygen O2 and made available as the so-called raw EDC;        a fractionating EDC purification in which the secondary products formed in the oxychlorination and EDC pyrolysis sections are removed from the raw EDC and the recycle EDC returned from the fractionation section in order to obtain a so-called feed EDC suitable for use in the EDC pyrolysis section, the use of the reaction heat developed in the direct chlorination of the EDC distillation being the main aim of the invention;        an EDC pyrolysis in which the pure EDC is combined with the feed EDC and in which the EDC mixture called crackable EDC is then thermally cracked, the cracked gas obtained consisting of VCM, hydrogen chloride HCl and non-reacted EDC as well as by-products;        a VCM fractionation in which the desired pure VCM product is separated from the cracked gas while the other essential substances, viz. HCl and non-reacted EDC contained in the cracked gas are recoverd specifically and returned as recycle HCl or recycle EDC in the balanced VCM process.        
The accompanying substances formed in the cracked gas during the EDC pyrolysis have a detrimental effect on the purity of the VCM product. Any VCM purification by removing the accompanying substances is a relatively expensive method. Hence, users of the balanced VCM process are working hard to reduce the costs incurred for the VCM purification in the fractionation. One of the considerations involved is to limit the types and quantities of unwanted accompanying substances in the EDC pyrolysis. This inevitably leads to the requirement that the content of impurities in the crackable EDC used for EDC pyrolysis should be as low as possible. The impurities contained in the crackable EDC partly form during the production of pure EDC and partly during the admixture of recycle EDC. Some of the impurities contained in the crackable EDC constitute precursors in the EDC pyrolysis and initiate the formation of additional accompanying substances in the cracked gas. The aim, therefore, is to minimise the amount of precursors penetrating via the crackable EDC into the EDC pyrolysis.
Since the realisation of the balanced VCM process concept many suggestions were submitted to avoid or remove the associated and detrimental by-products and/or the accompanying substances. It is known that part of the EDC required for the EDC pyrolysis is produced in the direct chlorination process by means of a reaction of ethene C2H4 with chlorine Cl2 to obtain liquid EDC. A high cooling capacity is required to maintain the reaction temperature since the reaction is exothermal. The direct chlorination reaction takes place in a circulated stream of the reaction product EDC in the presence of a Lewis acid (in most cases: iron (III) chloride) and an inhibitor (in most cases: oxygen). The distinguishing feature of the known reaction systems is the way in which circulation is accomplished, i.e. there are systems with natural circulation and systems with forced circulation.
Such a system with natural circulation is for instance described in DE 24 27 045. In this case, the reactants enter at the lower end of the riser of a loop-type reactor with natural circulation. The reactor inventory starts boiling in the upper part of the riser and the vapourous reaction products are directly piped to the bottom of a rectification column for direct heating of the latter.
The EDC reaction fluid may also be conveyed in a forced circulation system. A typical example of such a process is for instance described in DE 40 29.314 A1. The chlorine is taken in by an injector-type jet-of-liquid gas compressor and part of the chlorine dissolves in the reaction fluid. A downstream conventional gas header serves to admix the ethene by way of large gas bubbles. The complete stream then flows through a static mixer so that large gas bubbles are dispersed in order to facilitate the dissolution and the associated reaction. This type of system precludes any boiling of the reactor inventory, the EDC produced being withdrawn in the gaseous phase by flash evaporation of a part stream of the EDC reaction fluid. The heat recovery is accomplished in this system by means of the sensible heat contained in the EDC reaction fluid as described, for example, in EP 0 075 742 B1. The circulated liquid EDC stream passes through one or several thermosiphon reboilers which deliver heat for the column bottom, thereby transferring sensible heat to the boiling contents of the column bottom.
Patent DE 4029 314 A1 describes a direct chlorination process in which the overall content of all chlorinated by-products in the EDC produced in the loop-type reactor by means of a NaFeCl4 catalyst is below the value of 500 ppm. This EDC purity grade is normally appropriate for feeding the EDC to the pyrolysis unit without any intermediate purification step. Such a purity grade is of utmost importance to suppress, on the one hand, any side reaction which might cause fouling of the pyrolysis tubes and, on the other hand, to produce VCM of high purity. It is therefore necessary that the EDC left unreacted in the pyrolysis, the so-called recycle EDC and the so-called raw EDC leaving the oxychlorination be purified in a distillative EDC fractionation with high energy input prior to feeding them to the EDC pyrolysis. If the EDC impurity content is specified below 500 ppm it is also necessary to provide an additional distillative purification step for the pure EDC leaving the direct chlorination.
As a rule, all EDC streams are purified by way of distillation, hence the so-called EDC distillation. To achieve EDC purification the raw EDC from the oxychlorination and the non-reacted EDC from the EDC pyrolysis undergo purification in an EDC distillation with high energy input. If required, it is also possible to purify the pure EDC from the direct chlorination together with the raw EDC and the recycle EDC in the EDC distillation section. The EDC stream to be purified is first sent to a fractionation column, hereinafter referred to as “light ends column” to remove the water and light ends; the partly purified EDC which still contains heavy ends is withdrawn as bottom stream from the light ends column and piped to a fractionation column for the separation of heavy ends, hereinafter referred to as “heavy ends column”. The light ends column may be replaced by a series of separate columns.
The non-reacted recycle EDC from the EDC pyrolysis also contains heavy ends and is consequently fed to the heavy ends column. All process streams fed to the high ends column undergo distillation therein. A first part stream of EDC vapours freed to a large extent from heavy ends is withdrawn from the head of the heavy ends column, flows through the heat exchanger in which the vapours condense so that pure liquid EDC is obtained. The heavy ends are concentrated in the heavy ends column bottom. It is possible to purify the bottom discharge stream of the heavy ends column more intensely by feeding said stream to a second heavy ends column referred to as “vacuum column”. A second part stream of purified EDC vapours freed to a large extent from the heavy end is withdrawn from the head of the heavy ends column, flows through the heat exchanger in which the vapours condense so that pure liquid EDC is obtained. Upon being combined both streams form the crackable EDC. The bottom discharge stream from the vacuum column essentially consists of heavy ends and a small portion of EDC and must be disposed of.
Practical experience has shown that the direct chlorination reactor ranges among the largest consumers of coolant and the heavy ends column and vacuum column among the largest consumers of heating energy within an EDC/VCM plant operated by the balanced VCM process. On the basis of economic considerations, different concepts have been framed to reduce energy consumption, said concepts focusing on the heating of the columns. These concepts either make use of the reaction heat obtained in the direct chlorination for direct or indirect heating of the EDC distillation section or they only save comparably lower-valent heating energy by application of the rectification basics, including the compression of vapours and the distillative EDC purification. However, the process concepts suggested involve the disadvantages described in the following paragraphs.
As regards the direct heating method, the product vapours formed in the direct chlorination reactor are directly fed to the heavy ends column bottom as, for example, described in patents DE 29 35 884 and DE 24 27 045. The diameter of the high ends column and the surface area of its reflux condenser become very large because the multiple amount of EDC compared with the amount of EDC newly formed in the direct chlorination unit, will evaporate in the exothermal boiling reaction when no external cooling is provided and because they will need rectification in the heavy ends column in addition to the other EDC streams. As liquid EDC—also the multiple of the amount of the EDC newly formed in the direct chlorination reactor—must at the same time be returned from the bottom of the heavy-ends column to the reactor, the reactor will automatically reach an elevated level of higher-boiling by-products which, on the one hand, has a detrimental effect on the catalyst efficiency and, on the other hand, promotes the formation of by-products, finally resulting in a deteriorated yield.
On account of the process correlations described above, the reaction heat obtained in the direct chlorination unit is also unsuitable for the direct heating of the vacuum column of the EDC purification since the concentration of heavy ends in the bottom is 90% which exceeds the level prevailing in the heavy ends column.
There are two options for indirect heating: either liquid or vaporous EDC. The consequences involved are described in DE 196 41 562, DE 41 33 810 A1, DE 40 39 960 A1, DE 36 04 968 and EP 0 075 742 B1, details as follows:                If thermosiphon reboilers are used in the interconnected columns of the process stream system involved, adequate heating requires that an operational temperature difference of approx. 20 to 25° C. be ensured. The reaction temperature prevailing in the reactor must be raised considerably compared to the temperatures in the bottom of the columns involved in EDC purification in order to permit indirect reaction heat transfer to the thermosiphon reboilers. The required rise in temperature entails a deterioration of the EDC yield and an enhancement of by-product formation, and it will thus increase the amount of substances to be removed by distillation and the amount of heat required for the distillation.        The reaction heat developed in the reactor can also be transferred to heaters in the bottom of the interconnected EDC purification columns, even at relatively low reactions temperatures, provided that their operating pressure level is lowered to a value which, as a rule, is below the atmospheric pressure. This will inevitably lead to a larger diameter of the heavy-ends column and to a larger size of the reflux condenser which will have a detrimental effect on the process economy.        Patents EP 0 075 742 B1 and DE 41 33 810 A1 as well as this invention provide for the utilisation of liquid EDC from the direct chlorinations reactor but not for a use of the enthalpy of the vaporous EDC.        Patents DE 196 41 562 and DE 36 04 968 as well as this invention provide for the utilisation of the vaporous EDC and not of the liquid EDC.        